The final phase of this battle will require you to enter this large room. Head on to your left. It would be a good idea to hide behind one of the pillars, however you won't be fully protected there. Make sure that you've killed everyone. Collect the ammunition, especially since there's not going to be too many clips to take. Also, save your current progress.
The only available passageway can be found to the left of the reception desk. Head on to a small door. You'll find yourself inside the armory. Go ahead and open of the weapon cases. You'll be able to collect a few shotguns. If you don't want any more ammunition, head on to the second door. Your main target area is to your left. Sadly, this room is going to be heavily guarded. Try killing at least a few soldiers from here.
Most of the enemy soldiers will be using the upper balconies, so you may have some problems killing them. You can choose here between using a time stop function or zooming in the view on them. Try making a few steps forward. Sadly, this means more trouble, because reinforcements will arrive in the server room (screen). Once again, you will have to watch out for the upper platforms. Also, remember that you can always safely head back to the armory.
Enter the server room once again and turn right. I'm sure that you won't have any major problems finding the main computer (screen). Use this console in order to fulfill current mission objective. Sadly, you won't get necessary data from here. Go back to the armory right away, because more enemy soldiers are heading towards the armory. You will also be given a new task.
New mission objective: Access central mainframe in top floor control room
I guess you won't be surprised by the fact that you'll have to eliminate a few additional enemy units. Be careful, though. One of the enemy soldiers will appear near your position. Make sure that you've killed him right away. Once he's gone, secure all of the surrounding upper platforms. There's only one door here (screen), however it's going to be guarded, so open it carefully.
Now you will have to use the stairs to get to the upper level of the server room. You shouldn't encounter any additional units here. This gives you a great opportunity to collect all weapons from the eliminated soldiers. Once this is done, head on to a hole in the wall (screen). Enter it and start moving left. It would be a good idea to save your progress there.
There's only one passageway here, so you shouldn't have any problems finding the exit. Also, you won't have to be stealth here. You will come across a ladder. Obviously you must use it. This new room isn't guarded by anyone, so go ahead and head on to the next one. You will end up inside a long corridor (screen). Choose a rifle from the inventory and save your game.